Please answer the following questions 1. The finding that DNA is commonly underw
ID: 268809 • Letter: P
Question
Please answer the following questions
1. The finding that DNA is commonly underwound was a surprise. Predictions based on the B form of closed, circular DNA suggest that underwinding serves what purpose? A. It facilitates supercoiling B. It prevents strain on the helix and strand breaks. C. It promotes more rapid rates of endonuclease activity D. It prevents supercoiling and promotes spontaneous strand separation. E. It tends to prevent common structural mutations such as thymine dimers 2. What is the fundamental biochemical difference in the function of Type I versus Type II topoisomerases? A. whether they change linking number or retain the original linking number B. whether they break one DNA strand or botlh C. whether they relax DNA or supercoil DNA as a part of the enzyme mechanism D. whether they change the writhe or the twist of the DNA E. whether they have only phosphodiesterase activity, or both phospodiesterase and ligase activities 3. How are histones regulated? A. by acetylation B. by phosphorylation C. by sumoylation D. by ubiquitination E. All of the answers are correct 4. In what location do you expect to find single-stranded siRNA? A. attached to the major groove of DNA by hydrogen bonds B. bound to an operon repressor site C. bound to RNA polymerase D. bound to the small ribosomal subunit E. bound to mRNAExplanation / Answer
1. DNA molecules purified from both bacteria and eukaryotes are usually negatively supercoiled. Negative supercoils act as a store of free energy that aids in processes that require strand separation, such as DNA replication and transcription. Thus, strand separation can be accomplished more easily in negatively supercoiled DNA than in relaxed DNA. Option C) is correct.
2. Type II topoisomerases change the linking number in steps of two by making transient double-strand
breaks in the DNA. Type I topoisomerases change the linking number of DNA in steps of one by making transient single-strand breaks in the DNA. Option B) is correct.
3. Histone modification is a post-translational modification. Histone proteins can be acetylated, methylated, phosphorylated, sumoylated or ubiquitinylated. Option E) is correct.
4. siRNA stands for small interefering RNA. It is a synthetic RNA duplex made from double stranded RNA precursors which targets an mRNA for degradation. Both siRNAs and miRNAs direct a complex called RISC (RNA-induced silencing complex) to repress genes by attacking and digesting mRNA that has homology ; or interfering with translation of those mRNAs; or by directing chromatin modifying enzymes to the promoters that direct expression of those mRNAs. Option E) is correct.